ABSTRACT
Hydrogen gas (H2) is produced by bacteria both in anaerobic and aerobic environ ments [28]. The classes of anaerobic bacteria metabolizing H2 include methanogenic archaea, acetogenic bacteria, sulfate-and nitrate-reducing bacteria, as well as some hyperthermophilic bacteria. In these cases, fermentation of organic substrates (pro teins, nucleic acids, sugars, and lipids) yields H2. Reduction of H+ to H2 gas is also used by fermenting bacteria to dispose of excess reducing equivalents and to preserve a suitable redox potential in the cell. The H2 gas is subsequently utilized as a lowpotential reductant by bacteria living in the same environment, and its oxidation is usually coupled to the reduction of nitrate, sulfate, carbon dioxide, and fumarate, leading to ATP production.
